1. Field of the Invention
The present invention relates to a method for alignment of circuit patterns on a double sided opaque substrate or semiconductor, such as a silicon wafer, during the processing of integrated circuits. More particularly, the invention relates to a photolithographical method for alignment of patterns from the front side to the back side of the substrate or semiconductor wafer.
2. Description of the Prior Art
a. Background
Presently, most integrated circuits are produced using silicon wafers which are polished on only one side. However, there are specific applications which require the use of photolithography on the front and back of the substrate. There are methods available that will align the patterns on both sides of the substrate using an infrared light to see through the substrate. Another method for double sided alignment requires the use of a fixture which holds two masks in alignment with each other. The substrate is placed between two masks and imaged. These methods require specially developed equipment to do the alignment. Because this equipment is expensive, most facilities cannot justify the cost.
Front-to-back substrate alignment is a problem for the industry because of the cost and limited availability of the specialized equipment that is needed to easily perform this task. The user of an optical contact printer cannot see through the substrate to align the pattern on the front of the substrate to that on the back.
In optical contact printing, therefore, problems with substrate rotation and displacement arise when the front and back patterns must be aligned.
b. Specific Prior Art
U.S. Pat. No. 3,937,579 to Schmidt discloses a complete apparatus for the alignment of both sides of the wafer. The apparatus is similar to present contact mask aligners but provides for the insertion of two masks in a specially designed mask aligner with the wafer sandwiched between them. The two masks are then aligned to each other and the wafer.
U.S. Pat. No. 4,189,820 to Slack discloses a method where two masks and a wafer are sandwiched together and held. This sandwich is then exposed on both sides.
U.S. Pat. No. 4,323,327 to Slack discloses the use of a pivoting mechanism for the alignment of two masks to the wafer.
U.S. Pat. No. 4,534,804 to Cade describes the use of a laser beam to create a defect inside of the substrate. When the substrate is heated the mark moves from the inside of the substrate to the outside thus creating an alignment mark.
3. Advantages Over the Prior Art
The above prior art requires that the wafer either be sandwiched and aligned to the two masks or that special equipment be used to accomplish the alignment. The present method does not require special equipment. It allows a standard mask aligner to align a standard photomask with special alignment marks by windows and guides and facilitates alignment for both sides of the substrate in a simplified and easily performed manner. A window is an area where the chrome surface of the photomask is removed leaving the glass substrate uncovered. These alignment marks are placed on the first mask that is used. This mask can also have other pattern data on it so that an extra alignment mask is not required.
To solve these problems, mask alignment methods have been designed in the Semiconductor Engineering and Materials Technology (SEMT) Facility of the Army Research Laboratory (ARL).rarw.. The present invention discloses the results of two projects conducted at ARL which projects required accurately aligned photolithography on both sides of double-polished silicon substrates. The first project involved development of a fabrication method for a semiconductor chip for an in-house research program. This method is termed the lower alignment accuracy method. This method was successfully used to produce over 4000 test chips. The second project involved development of a fabrication method in which floating cantilever beams were used as accelerometer switches. This method is termed the higher alignment accuracy method. Several hundred cantilever accelerometers were produced using this method. These methods allow the user to contact print quickly with a front-to-back alignment accuracy of better than 50 microns without the use of expensive through-the-substrate alignment equipment. For limited use, the methods developed in the SEMT laboratory enable double sided-pattern alignment using standard single sided imaging equipment.
The present invention provides a double sided substrate alignment method using a minimum of standard imaging equipment as oppose to the prior arts use of expensive specially developed equipment to perform the double sided alignment. A front-to-back substrate alignment accuracy of better than 25 microns is achieved with applicants higher alignment accuracy method using a standard optical contact printer.